Photovoltaic solar cells essentially comprise a semi-conductor substrate having a shallow P-N junction formed adjacent the front surface thereof. The cells require electrical contacts (also called "electrodes") on both their front and rear sides in order to be able to recover an electrical current when the cells are exposed to solar radiation. The contact on the front of the solar cell is generally made in the form of a grid, comprising a plurality of narrow, elongate parallel fingers and at least one elongate busbar that intersects the fingers at a right angle. The width, number and spacing of the fingers are set so as to expose an optimum area of the front surface of the cell to incident solar radiation. The busbar serves as the solder bonding terminal for the grid contact whereby the contact is interconnected with other cells.
In order to improve the conversion efficiency of the cells, a thin anti-reflection ("AR") coating consisting of a material such as silicon nitride is provided on the front side of each cell. The AR coating may be applied before or after the grid electrode has been formed.
Commercial acceptance of solar cells is conditioned upon the ability to produce reliable, efficient and long-lasting cells and solar cell modules. As used herein, the term "solar cell module" means a plurality of solar cells that are interconnected electrically and physically so as to form a discrete array that provides a predetermined voltage output, e.g., a module may consist of 216 cells connected together so as to have a total power output of 220 watts. Typically in such a module the cells are arranged in a rectangular array comprising 12 strings each consisting of 18 cells, with the cells in each string being connected in series and the strings being connected in parallel. The cells in each string are interconnected by copper connecting strips (also called "tabs") that are bonded to the front and back electrodes. A selected number of such modules may be connected together to form a solar panel having a desired power output.
The reliability and efficiency of solar cells are affected by the nature and quality of their front and rear contacts. Accordingly, much effort has been directed to developing reliable, low resistance, solderable contacts and improving the techniques of making the contacts so as to reduce breakage and thereby increase production yield of solar cells.
Aluminum, because of its low cost and good electrical conductivity, is the most common material used to fabricate the back contacts of solar cells. Typically, the aluminum back contact is made by coating the rear side of a silicon substrate with an aluminum paste (also called an "ink"). The terms "paste" and "ink" are used interchangeably to describe contact-forming liquid compositions comprising metal particles disposed in an organic liquid vehicle. Therefore, in the context of this invention, both terms are used to describe the viscous aluminum and silver compositions that are used to form solar cell contacts. The aluminum pastes are fired so as to remove the organic vehicle by evaporation and/or pyrolysis and cause the metal particles to alloy with the silicon substrate and thereby form an ohmic aluminum/silicon contact. Unfortunately, aluminum tends to form a surface oxide when exposed to air. That surface oxide increases contact resistance and also inhibits direct soldering.
Various methods have been used for overcoming the aluminum soldering problem. A currently favored technique involves first forming an aluminum contact on the back of the solar cell substrate that is discontinuous in the sense that it defines a plurality of openings ("windows") through which the underlying silicon substrate is exposed, and then applying a suitable silver paste to those areas of the back surface of the substrate that are exposed by the windows. These areas of silver paste are then fired to form a plurality of silver "soldering pads". Preferably the silver soldering pads overlap the surrounding aluminum contact. These soldering pads are used as solder bonding sites for cell interconnecting tabs in the form of tin-coated copper ribbon which are used to interconnect aluminum contact to adjacent solar cells. This technique is described in copending U.S. patent application Ser. No. 561,101, filed Aug. 1, 1990 by Frank Bottari et al for "Method Of Applying Metallized contacts To A Solar Cell". To the extent necessary, the information disclosed in said U.S. application Ser. No. 561,101 is incorporated herein by reference thereto.
The grid contacts are commonly formed of silver or nickel. One prior art method of forming the front grid contact involves applying a conductive silver metal paste onto the front surface of a solar cell substrate in a clearly defined grid electrode pattern, firing that paste so as to form a bonded ohmic contact, and then applying the AR coating to the front surface of the solar cell substrate. Another common procedure is to first form the AR coating, then etch away portions of that coating so as to expose portions of the front surface of the solar cell substrate in a grid electrode pattern, and thereafter deposit a paste or otherwise form the front contact in those regions where the AR coating has been removed.
Still a third approach is the so-called "fired through" method which consists of the following steps: (1) forming an AR coating on the front surface of the solar cell substrate, (2) applying a coating of a paste comprising metal particles and a glass frit onto the AR coating in a predetermined pattern corresponding to the configuration of the desired grid electrode, and (3) firing the paste at a temperature and for a time sufficient to cause the metal/glass frit composition to dissolve the underlying AR coating and form an ohmic contact with the underlying front surface of the solar cell substrate. The "fired through" method of forming contacts is illustrated by PCT patent application Publication No. WO 89/12321, published 14 Dec. 1989, based on U.S. application Ser. No. 205,304, filed 10 Jun. 1988 by Jack Hanoka for "An Improved Method of Fabricating Contacts For Solar Cells". The same concept of firing metal contacts through an AR coating is further described in U.S. Pat. No. 4,737,197, issued to Y. Nagahara et al for "Solar Cell With Metal Paste Contact". The teachings of those documents is incorporated herein by reference thereto.
Attempts to reduce the cost of manufacturing solar cells have involved investigation and use of a number of coating techniques for applying metal-containing conductive inks to the solar cell to form contacts, notably screen printing, pad printing, and direct writing. The pad printing technique is preferred for forming aluminum back contacts with windows filled with silver soldering pads because it permits formation of contacts at a relatively low cost, with high throughput rates and very low substrate breakage rates. The direct write technique is preferred for applying a silver paste to the front side of the solar cell substrate to form the busbars and fingers of the grid electrode. This technique is illustrated and described in copending U.S. application Ser. No. 666,334 of Jack I. Hanoka et al, filed 7 Mar. 1991 for "Method And Apparatus For Forming Contacts". The teachings of that application are incorporated herein by reference thereto.
In the course of manufacture and in many common applications, the photovoltaic cells are subjected to continuous high temperatures or else to thermal cycles at regular or irregular intervals. For example, in the ethylene vinyl acetate (EVA) lamination procedure that typically follows cell stringing (interconnecting) in the manufacture of multi-cell modules, the cells are subjected to temperatures as high as about 150.degree. C. for about 45-60 minutes. Also, when used in the production of solar energy, the cells will heat up during a cycle of exposure to sunlight and then cool down again to ambient temperatures at night. In other applications, the heating and cooling cycles may be much more frequent. Accordingly, an important requirement of such cells is the ability to withstand thermal aging, particularly with respect to their solder connections.
The twin requirements of high reliability and efficiency have placed significant demands upon the contacts metallization system. In this connection, it is to be appreciated that an important component of the performance of solar cell modules is the reliability of the bonds between the cell contacts and the solar cell substrates, which are subject to thermal aging and also peeling forces applied by the cell-coupling tabs that interconnect the cells in a module.
Prior art silicon photovoltaic cell modules incorporating silver electrical front contacts and aluminum rear contacts with silver soldering pads, with copper cell-coupling tabs soldered to those contacts and pads, tend to show poor mechanical reliability of their solder bonds when subjected to thermal aging. It has been found that the bond reliability factor is especially critical for the grid contact. Specifically, it is known that the strength of solder bonds made to silver/glass thick films on silicon using 63% tin/37% lead or 62% tin/36% lead/2% silver solders degrades by more than 80% upon exposure to temperatures of 150.degree. C. for one hour. As noted above, exposure to such temperatures for up to one hour is typically required to laminate the cells to glass in the manufacture of photovoltaic modules. Consequently, the substrate/solder/tab bonds in modules made with such solders are inherently weak. Stress testing of modules made in this manner indicate that their performance degrades relatively rapidly under conditions that produce mechanical loading on the module, including changes in temperature which can be expected to occur in typical applications.
The literature in this field also suggests that the problem of thermal instability of contacts with solder bonds as described above, may be caused, at least in part, by the formation of intermetallic compounds between tin and silver because such compounds are known to be brittle and weak. While heating promotes formation of such components, it is believed that they tend to form slowly even at room temperature. There has been some belief that these brittle compounds promote the metallization failure at low stress levels.
Recently, it was discovered that improved contact reliability can be achieved by bonding the copper tabs to the silver grid contacts and soldering pads by using a solder paste with a combination of tin and silver ranging from about 96% tin/4% silver to about 98% tin/2% silver, with the paste preferably incorporating one or more compatible, volatile fluxing agents. This invention is disclosed and claimed in U.S. Pat. No. 5,074,920, issued to Ronald C. Gonsiorawski et al for "Photovoltaic Cells With Improved Thermal Stability", which is incorporated herein by reference thereto. An example of a solder selected according to the teachings of Gonsiorawski et al is "Xersin 2005", which is a solder paste comprising approximately 96% tin and 4% silver in a synthetic flux, manufactured by Multicore Corp., a company having United States offices in Westbury, N.Y. To the extent necessary, the teachings of Gonsiorawski et al U.S. Pat. No. 5,074,920 are incorporated herein by reference thereto.
In the development of this invention, efforts were directed to determining whether or not selected silver pastes could improve grid electrode bond reliability. It was observed that the grid electrode bond reliability problem was most acute in the regions where the tabs are soldered to the bus bars of the grid contacts. A severe reduction in peel strength in those regions was observed when the cell contacts were thermally soaked (i.e., heated to a predetermined temperature for a measured time) and then tested for peel strength. The thermal aging effect leading to reduced peel strength was investigated as to the role of the solder chemistry and also the physical and chemical properties of different silver pastes. The investigation has lead to the belief that the failure mechanism is related to surface diffusion of tin from the solder under the driving force of heat. In-diffusion of tin deep into the bulk of the silver busbar is believed to cause the busbar structure to swell, putting tensile stress on the glass component of the busbar. Tensile stress reduces the fracture strength of the busbar, resulting in lower contact bond peel strength.
One result of the investigation was the discovery, disclosed by the Gonsiorawski et al U.S. Pat. No. 5,074,920, supra, that the thermal aging effect could be reduced by changing solder composition, since the role of solder chemistry in the fracture mechanism is related to the eutectic temperature of the solder. Tin surface diffusion at temperatures close to the solder eutectic temperature is significant, causing the swelling and weakened bonds. As disclosed in said U.S. Pat. No. 5,074,920 of Ronald C. Gonsiorawski et al, improved temperature stability can be achieved if the standard 62% tin/30% lead/2% silver (which has a eutectic temperature of about 179 degrees C.) is replaced by a solder having a composition in the range of 96% tin-4% silver to 98% tin-2% silver. The 96% tin-4% silver solder has a eutectic temperature approximately 40 degrees C. higher than the standard 62% tin-36% lead-2% silver solder, and this higher eutectic temperature improves resistance to thermal aging of the contact bonds.
A second result of the investigation is the present invention. It was postulated that improved metallization reliability for the grid electrode could be achieved by using a silver paste which, after being fired, would be more resistant to in-diffusion of tin. The present invention arises from and comprises the discovery that the form and size of the particles in the silver paste or ink has an effect on bond stability under thermal aging. The present invention comprises the concept that if substantially all of the metal particles in the silver paste have a generally spherical or round form, the reliability of the bond between the contacts and the underlying substrate is greatly improved in the region of the solder connections between the contacts and the tabs that interconnect adjacent cells in a module.